A series of powerful storms is set to soak California over the next week, bringing heavy rains, flooding risk in some creeks and 10 feet or more of new snow to the Sierra Nevada — the latest sign that the stranglehold of the state’s five-year drought is significantly weakening. At least three storms are forecast to crash in from the Pacific Ocean, weather forecasters said Tuesday, dumping 3 to 4 inches of rain on most Bay Area cities by next Monday, and 8 to 10 inches in the Santa Cruz Mountains, Marin hills and Big Sur coast.